An image sensor converts a visual image to digital data that may be represented by a picture. The image sensor comprises an array of pixels, which are unit devices for the conversion of the visual image into digital data. Digital cameras and optical imaging devices employ an image sensor. Image sensors include charge-coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) image sensors.
While CMOS image sensors have been more recently developed compared to the CCDs, CMOS image sensors provide an advantage of lower power consumption, smaller size, and faster data processing than CCDs as well as direct digital output that is not available in CCDs. Also, CMOS image sensors have lower manufacturing cost compared with the CCDs since many standard semiconductor manufacturing processes may be employed to manufacture CMOS image sensors. For these reasons, commercial employment of CMOS image sensors has been steadily increasing in recent years.
The CMOS image sensor unit cell comprises a pixel area and a logic area. The pixel area typically contains one of each type of active pixel, e.g., a red pixel having a first photodiode underneath a red optical filter, a green pixel having a second photodiode underneath a green optical filter, and a blue pixel having a third photodiode underneath a blue optical filter. The pixel area may also contain a dark pixel, which is covered with an opaque layer such as an aluminum light shield and employed to measure a background output level in the absence of illumination so that the output of other pixels may be referenced and calibrated. U.S. Pat. No. 6,750,912 to Tennant et al., which is incorporated herein by reference, describes use of a dark pixel.
Each photodiode generates charges upon exposure to light. Logic devices connected to the photodiode detect and amplify the charges to generate signals proportional to the incident light. Each pixel comprises at least one photodiode to convert incident photons into electrical charges. Since the area of the photodiode is less than the area of each pixel, a convex-top flat-bottom lens, i.e., an optical lens having a convex surface at the top and a substantially flat surface at the bottom, is typically formed over each photodiode so that photons incident upon a convex-top flat-bottom lens are focused onto the photodiode located underneath. Typically, a color filter is formed between the convex-top flat-bottom lens and the photodiodes to make each pixel color-sensitive, i.e., responsive to photons within a certain wavelength range. The convex-top flat-bottom lens thus acts to focus light over a wide area onto the area of the photodiodes.
The efficiency of each pixel depends on the amount of transmitted light through the light path as well as the quality of the lens system thereabove in focusing the transmitted light. Many technological advances in the lens system, such as composite lens systems having multiple lenses in the light path, have been made to provide more effective focusing of lights that impinge on the outer surface of a pixel area on a semiconductor chip. Transmission of light through the light paths is maximized by providing as few optical interfaces in the light path, as well as employing material providing as small a difference as possible at each optical interface. In many cases, unnecessary optical interfaces are eliminated from a semiconductor to improve light transmission. In some cases, optical interfaces are eliminated even at the expense of adverse effects such as degradation of process control or reliability. For example, openings in cap dielectric layers are formed within a pixel area despite the degraded protection of underlying metal lines and/or degraded process uniformity and topography in metal interconnect structures thereabove.
Despite such structural modifications introduced into a pixel structure, loss of light due to reflection in the optical path is still significant, and thus, is a limiting factor on the efficiency of image sensor pixel structures.
In view of the above, there is a need for a structure reducing reflection of light in the optical path of a CMOS image sensor pixel, thereby enhancing transmission of light to a photodiode and increasing the overall efficiency of the image sensor pixel, and methods of manufacturing the same.